1. Field of the Invention
The present invention relates to a developing apparatus for developing an electrostatic image that has been formed through an electrophotographic printing method or an electrostatic recording method on an image bearing member, in particular, having a developer carrying member.
2. Description of the Related Art
Up to now, in an image forming apparatus such as an electrophotographic copying machine, a powder cloud method, a cascade method, and a magnetic brush method have been known as methods employed for developing apparatuses that are applied to the image forming apparatuses. Among those, in a case of the magnetic brush method of a two-component developing system, a two-component developer mixedly containing magnetic carriers and toner therein is used as the developer. Then, the developer is attracted by magnetic field generating means and stands like the ears of rice in a shape of brush on a magnetic pole portion, and an electrostatic latent image on a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) which serves as the image bearing member is rubbed by the developer to thereby develop and form an image. In this event, because the magnetic carrier per se in the developer serves as a soft developing electrode, it is possible to make the toner adhere to the electrostatic latent image in proportion to charge density of the electrostatic latent image. In other words, the magnetic brush method of the two-component developing system is suitable for a reproduction of a gradation image. Also, the magnetic brush method of the two-component developing system has a feature that the developing apparatus per se can be downsized.
The magnetic brush developing method using a development sleeve which is the developer carrying member is generalized as a magnetic brush developing apparatus of the two-component developing system.
In the magnetic brush developing method, in order to efficiently develop the electrostatic latent image on the photosensitive drum, the two-component developer containing magnetic powders, for example, magnetic carriers which are ferrite or the like, and toner in which pigment is dispersed in a resin are agitated and mixed together. The agitation and mixture of the developer allow the toner to carry electric charges through frictional charge attributable to the friction between the developers. On the other hand, the developer is held by the development sleeve serving as a hollow cylindrical developer carrying member which has the magnetic pole therein and is made of a nonmagnetic material. The developer that is held by the development sleeve is transported to a development area that faces the photosensitive drum from a developer container by using the development sleeve. The developer that has been transported to the development area is stood like the ears of rice through an action of the magnetic field in the development area, and rubs the surface of the photosensitive drum. As a result, the electrostatic latent image that has been formed on the photosensitive drum is developed by the developer.
The two-component magnetic brush developing method using the development sleeve is mainly employed in various products, typical examples of which include a monochrome digital copying machine and a full color copying machine requiring a high image quality.
Up to now, in a case where a rotation movement speed of the photosensitive drum is relatively low, that is, in a case of a copying machine having relatively low operating speed, a sufficient and excellent developed image is obtained with a short development period. For that reason, an excellent image is obtained even when a number of the development sleeves is one.
However, in a case where the rotation movement rate of the photosensitive drum becomes higher in a course of a demand for increasing the operating speed of the copying machine in recent years, it is not always possible that proper image formation can be conducted by one development sleeve.
As a countermeasure against the problem described above, there is a method in which peripheral speed of the development sleeve is increased to enhance development efficiency. However, a centrifugal force that is exerted on the developer constituting the magnetic brush becomes larger as the peripheral speed of the development sleeve increases. This increases a scattering rate of the developer, induces contamination of an interior of the copying machine, and deteriorates functionalities of the apparatus.
Under the above-mentioned circumstances, as another countermeasure, there has been proposed a so-called multi-stage magnetic brush developing method using two or more developer carrying members such as the development sleeves. That is, in the multi-stage magnetic brush developing method, a plurality of development sleeves are disposed in such a manner that peripheral surfaces thereof are brought in vicinity of each other so as to be adjacent to each other. Then, the developer is continuously transported through the respective peripheral surfaces, and the development period is extended to enhance development performance.
Now, an example of the developing apparatus of the multi-stage magnetic brush developing system having two conventional development sleeves is shown in FIG. 13.
A developing apparatus 104 includes a developer container 122 that is disposed in parallel to a photosensitive drum 101, and an interior of the developer container 122 is compartmented into a development chamber R1 and an agitation chamber R2 by a partition 123 that is in parallel to the photosensitive drum 101. A developer 120 into which the toner particles and the magnetic carriers are mixed together is housed in the development chamber R1 and the agitation chamber R2.
A transporting screw 124 is housed in the development chamber R1, and the transporting screw 124 transports the developer 120 along a longitudinal direction of the developer container 122 which is in parallel to the photosensitive drum 101 through rotational driving. A transporting screw 125 is housed in the agitation chamber R2, and transports the developer 120 along a longitudinal direction of the developer container 122 which is in parallel to the photosensitive drum 101 through rotation driving. A developer transporting direction of the transporting screw 125 is opposite to that of the transporting screw 124.
Openings 123a and 123b are defined in the partition 123 at a back side and a front side of FIG. 13 as can be understood with reference to FIG. 14. The developer 120 that has been transported by the transporting screw 124 is transferred to the transporting screw 125 from the opening 123a, and the developer 120 that has been transported by the transporting screw 125 is transferred to the transporting screw 124 from the opening 123b. 
An opening portion is defined at a portion of the developer container 122 in vicinity of the photosensitive drum 101, and two developer carrying members consisting of a first development sleeve 126 and a second development sleeve 128 which are made of a non-magnetic material are disposed in the opening portion. The first development sleeve 126 is disposed opposite to the photosensitive drum 101 to define a development area A1, and the second development sleeve 128 is disposed opposite to the photosensitive drum 101 to define a development area A2.
Of the two developer carrying members, the first development sleeve 126 that is disposed opposite to the photosensitive drum 101 at an upstream side in a rotating direction “a” of the photosensitive drum 101 rotates in a direction indicated by an arrow “b” (in a direction opposite to the rotating direction “a” of the photosensitive drum 101).
Also, in this example, a blade-shaped developer regulating member (layer thickness regulating blade) 121 is disposed at a top end of the opening portion of the developer container 122, that is, upstream of the development area A1 in the rotating direction of the development sleeve 126 in general. The development sleeve 126 carries and transports the developer 120 to the first development area A1 after the retained developer is regulated to an appropriate developer layer thickness by the layer thickness regulating blade 121.
A roller-shaped first magnetic field generating means (hereinafter referred to as “magnet roller”) 127 is fixed and disposed within the development sleeve 126. The first magnet roller 127 has a development magnetic pole S1 that faces the first development area A1. The magnetic brush of the developer is formed through a development magnetic field that is developed in the first development area A1 by the development magnetic pole S1, and the magnetic brush comes into contact with the photosensitive drum 101 rotating in the direction indicated by the arrow “a” in the first development area A1 to develop the electrostatic latent image in the first development area A1.
The first magnet roller 127 has N1, S2, N2, and N3 poles in addition to the above-mentioned development magnetic pole S1, and the N2 pole and the N3 pole are identical in polarity with each other and adjacent to each other within the developer container 122 to develop a repulsive magnetic field, thus producing a barrier with respect to the developer 120.
In addition, the second development sleeve 128 that is a second developer carrying member is disposed below the first development sleeve 126 and at a downstream side in the rotating direction “a” of the photosensitive drum 101. Moreover, the second development sleeve 128 is disposed in an area substantially facing both the first development sleeve 126 and the photosensitive drum 101, and is also located rotatably in a direction indicated by an arrow “c” which is the same direction as that of the first development sleeve 126.
The second development sleeve 128 is made of a non-magnetic material, as with the first development sleeve 126, and a roller-shaped second magnet roller 129 that is a second magnetic field generating means is located in a non-rotating state in the interior of the second development sleeve 128. Also, the second magnet roller 129 has five poles consisting of magnetic poles S3, N4, S4, N5, and S5.
The developer 120 is transported in the stated order of N2→S2→N1→S1→N3 on the first development sleeve 126. Thereafter, the developer on the first development sleeve 126 is moved to the second development sleeve 128, and is transported in the stated order of S3→N4→S4→N5→S5 on the second development sleeve 128. In this example, the developer is transferred by the poles substantially facing each other and having the polarities different from each other (i.e., N3 pole and S3 pole). This is because in a case where the poles are identical in polarity with each other, the magnetic force lines are not produced and stable transfer cannot be conducted.
In the above-mentioned structure, the magnetic brush produced in the N4 pole comes into contact with the photosensitive drum 101 at an opposing portion of the second development sleeve 128 and the photosensitive drum 101, that is, the second development area A2. Then, the electrostatic latent image on the photosensitive drum 101 which has passed through the first development area A1 is further subjected to a second development process. In this way, the two development processes are conducted to achieve high development efficiency.
As described above, with the structure in which two development sleeves 126 and 128 are disposed, for example, even if the development period becomes shorter as the peripheral speed of the photosensitive drum 101 increases, high development efficiency can be achieved, thereby making it possible to excellently form an image without deterioration of the development density and occurrence of density unevenness.
Incidentally, the developer 120 within the developer container 122 is transferred to a portion of a bearing 140 of the development sleeves 126 and 128 shown in FIG. 15A along the surfaces of the development sleeves 126 and 128 by circulation within the developer container 122. For that reason, the developer enters the portion of the bearing 140 and stays within the bearing 140 to inhibit the function thereof. As a result, there is a case in which a smooth rotation of the development sleeves 126 and 128 is disabled, or the developer passes through the portion of the bearing 140, causing the developer to leak out from the developer container 122 or scatter.
To cope with the scattering of the developer from the end of the development sleeve, there has been proposed a method in which elastic seal members are fitted onto both ends of the development sleeves, and the ends of the seal members are sealed to prevent the toner from leaking.
However, in the above-mentioned seal structure, because the elastic sealing members are fitted onto the outer peripheral surfaces of the development sleeves under pressure, there arise such problems that a load on the development sleeves becomes large, and the sealing property is deteriorated due to the deterioration of the elastic seal members.
Under such the circumstances, there has been proposed a developing apparatus using a magnetically attractive toner or carrier, in which magnetic sealing is conducted by magnetic force generating means (for example, refer to JP 11-133750 A).
Shown in FIG. 15B is a structure in which a magnetized magnetic seal member MP is disposed on an opposing surface with respect to a surface of a development sleeve SL at a given gap to magnetically attract and hold the developer.
The above-mentioned magnetic seal structure is advantageous in that a rotation load of the development sleeve SL is reduced because the development sleeve SL and the magnetic seal member MP are out of contact, and that the lifetime is prolonged because the development sleeve SL and the magnetic seal member MP are not deteriorated due to the friction.
When a plate-shaped magnet is disposed as the magnetic seal member MP to surround the development sleeve SL in a non-contacting fashion, the magnetic brush is produced between a magnetic roller MR and a magnet MP within the development sleeve SL by using the developer, thereby making it possible to prevent the leakage of the developer. In the developing apparatus shown in FIG. 13, in a case of using a magnet plate having one surface of N pole and the other surface of S pole as the magnet MP, it is desirable that a surface having a pole different in polarity from a pole (N2 and N3, and S3 and S5) that produces the repulsive magnetic field of the magnet roller is a surface at the development sleeve side. In a case where the above-mentioned structure is not applied, the leakage of the developer in the longitudinal direction of the sleeve is liable to occur. The reasons will be described below.
A description will be given with reference to FIGS. 16A to 16C and FIGS. 17A to 17C. In a case where the repulsive magnetic field and the magnetic seal member are identical in polarity with each other while facing each other, the repulsive magnetic field is also produced between the repulsive magnetic field and the magnetic seal member. For that reason, as shown in FIG. 16A, the line of magnetic force of the magnetic seal is unintentionally bent toward the outside of the longitudinal direction of the development sleeve SL and extended. In this case, because the developer is arranged along the line of magnetic force as shown in FIG. 16B, the developer is extended toward the direction of the end of the development sleeve SL, and the developer is liable to leak in the direction of the end thereof.
FIG. 16C schematically shows a force that is applied to the magnetic carrier in an area surrounded by the magnetic seal member MP and the development sleeve SL. The arrows indicate a direction of force at that position, and a length of the arrow indicates a magnitude of the force.
In a case where the magnets have the same polarity facing each other, an area (where the direction of the force exerted on the magnet is inverted), in which there is substantially no magnetic force exerted on the magnetic carrier between the magnets, continuously exists in the longitudinal direction between the magnets. In FIG. 16C, an area in which there is substantially no magnetic force exerted on the magnetic carrier is designated by a mark O.
However, as shown in FIG. 16C, in a case where the area in which there is substantially no force exerted on the magnetic carrier continuously exists between the magnetic seal member MP and the development sleeve SL in the longitudinal direction, no magnetic carrier is attracted to the magnetic seal member MP or the magnet roller MR. For that reason, it is possible to leak the developer according to a flow indicated by a dotted arrow shown in FIG. 16C. As a result, the developer is liable to flow out of the development area, thereby making it impossible to exercise the excellent sealing property.
Under the circumstances, there has been proposed a structure in which the repulsive magnetic field and the magnetic seal member are different in polarity from each other while facing each other.
In other words, when the repulsive magnetic field and the magnetic seal member are different in polarity from each other while facing each other, since the line of magnetic force of the magnetic seal is extended toward the direction of the development sleeve as shown in FIG. 17A, it becomes difficult to extend the line of magnetic force of the magnetic seal toward the outside of the longitudinal direction. As a result, it becomes difficult for the developer to leak toward the direction of the end of the development sleeve. In this situation, the developer is extended toward the direction of the development sleeve as shown in FIG. 17B, and the magnetic brush is produced between the development sleeve SL and the magnetic seal member MP by the developer. The magnetic brush functions to seal the developer that is to leak toward the end direction, and it is further suppresses leakage of the developer.
On the other hand, in a case where the repulsive magnetic field and the magnetic seal member are identical in polarity with each other while facing each other as described above, as shown in FIG. 16B, the magnetic brush is not extended toward the development sleeve direction from the magnetic seal member MP. Therefore, there is an area in which no developer exists between the magnetic seal member MP and the development sleeve SL, and the developer is liable to leak.
FIG. 17C schematically shows a force that is exerted on the magnetic carrier in an area surrounded by the magnetic seal member MP and the development sleeve SL as in FIG. 16C.
In a case where the magnets are different in polarity from each other while facing each other, an area (indicated by a symbol “O”) in which there is substantially no magnetic force exerted on the magnetic carrier between the magnets exists, but does not continuously exist between the magnets. For that reason, the magnetic carrier in the area surrounded by the magnetic seal member MP and the development sleeve SL is always attracted to the magnetic seal member MP and the magnet roller MR during a process in which the magnetic carrier is moved in the direction of the end of the development sleeve. As a result, it is difficult to that the developer flows out of the development area, thereby making it possible to exercise the excellent seal property.
Also, there has been proposed a structure using magnets in which NS poles are magnetized to multiple magnetic poles on an inner peripheral surface as the magnetic seal member MP. In the above-mentioned structure, because the line of magnetic force is extended between the multiple magnetic poles of the magnetic seal member, it is difficult that the line of magnetic force is extended to the outside of the longitudinal direction of the development sleeve, whereby excellent property can be exercised.
In a case where the above-mentioned magnetic seal structure is applied to the magnetic brush developing apparatus having the two development sleeves shown in FIG. 13, there arise the following problems.
As shown in FIG. 18, in the case of using magnet plates 130 and 131 each having one surface of N pole and a back surface of S pole as the magnetic seal member, the upstream development sleeve 126 has an S pole surface that is different in polarity from the repulsive magnetic poles (N2 and N3) as the inner peripheral surface. Also, the downstream development sleeve 128 has an N pole surface that is different in polarity from the repulsive magnetic pole (S3 and S5) as the inner peripheral surface. From the viewpoint of the above-mentioned conventional art, the above-mentioned structure appears to suppress the leakage of the developer to the outside of the development area, and exert the excellent sealing property.
However, according to the inventors' study, it has been found that in the above-mentioned structure, the developer is leaked in the downstream sleeve rotating direction from a space between the upstream and downstream development sleeves 126 and 128 in the end of the development sleeve. The reasons will be described below.
Of the S3 pole and the S5 pole which produce the repulsive magnetic field of the downstream development sleeve 128, the S3 pole also serves as a delivery pole that receives the developer from the upstream development sleeve 126. As a result, because the upstream development sleeve 126 exists at an opposing portion of the S3 pole, the magnetic seal member 131 is capable of extending only up to the middle of the repulsive magnetic field as shown in FIG. 18. For that reason, the magnetic seal member 131 does not face the S3 pole of the delivery pole.
However, because the line of magnetic force is developed between the magnetic seal member 131 and the S3 pole of the delivery pole, a part of a developer that has been caught by the magnetic seal member 131 is attracted to the S3 pole of the delivery pole and moved. The developer that has been moved to the S3 pole from the magnetic seal member 131 at the end of the development sleeve has no magnetic seal member at a side opposite to the S3 pole, and leaks in the development sleeve end direction. The developer that has leaked out in the end direction is transported with the rotation of the development sleeve, and leaked from the space between the upstream and downstream development sleeves 126 and 128.
Similarly, in the case of a structure using a magnet, whose NS poles are magnetized to the multiple magnetic poles on the inner peripheral surface, as the magnetic seal member, because the line of magnetic force is formed between the magnetic seal member and the S3 pole of the delivery pole, the developer is leaked from a space between the upstream and downstream development sleeves as in the above-mentioned case.